The present invention relates to the art of magnetic resonance imaging. The invention is particularly applicable to the imaging of resonating hydrogen dipoles which are bound in part in water molecules and in part in lipid molecules and will be described with particular reference thereto. It is to be appreciated, however, that the present invention is applicable to correcting other types of images, creating images from other nuclei and the like. These concepts may also be utilized to perform other applications of chemical shift imaging where more than two shift species are important. This technique also produces a characterization of the magnetic field within a subject which may be used for spectroscopic corrections, shimming and the like.
When imaging human patients, hydrogen dipoles commonly present in water, lipid, and other molecules are in the image region. The gyromagnetic ratio of the hydrogen dipoles bound in water differs from the gyromagnetic ratio of hydrogen dipoles bound in lipids. This causes the lipid and water dipoles to precess at different resonance frequencies. This frequency difference causes corresponding errors in the image, such as a shifting of lipid information relative to water, blurring of the image, and the like.
Various techniques for water and lipid separation have been implemented, including selective saturation, multiple quantum, relaxation rate, and alternative chemical shift based techniques. In selective saturation techniques, radio frequency excitation signals are broadcast at separate frequencies. One corresponds to water and the other corresponds to lipid. Either the water or the lipid is selectively excited to saturation and the other is imaged conventionally.
The multiple quantum techniques relay upon the strong multiple quantum coherence between energy states of lipid molecules. The quantum coherence between energy states can be indirectly detected without interference from the single quantum water signals.
The relaxation rate techniques exploit the differences between water and lipid spin-spin or spin-lattice relaxation rates. In an inversion recovery sequence, the inversion time is chosen such that the lipid contributions are placed at the crossover point when the 90.degree. excitation pulse is applied. The net magnetization from the lipid is now zero so that the lipid contribution to the final image is nulled.
In normal spin-echo imaging, a resonance excitation pulse is applied to incite resonance in the hydrogen dipoles. Because the water and lipid dipoles are resonating at different frequencies, the resonance signal therefrom progressively dephase, i.e. become more out of phase. A 180.degree. inversion pulse is applied to reverse the spin direction such that the magnetic resonance signals begin to rephase, i.e. converge back together. The components come back into phase to produce an echo at a time interval after the inversion pulse which is equal to the timer interval between the excitation and inversion pulses. The signal is monitored started before the echo and continuing after the echo and transformed into an image representation.
Shifting the inversion pulse in time by a preselected duration corresponding to one quarter of the inverse of the lipid resonance frequency renders the water and lipid magnetization vectors 180.degree. out of phase during the sampling. Alternatively, the 90.degree. excitation pulse could be moved by twice this amount. However, if this is done, the effective TE of the experiment is changed. The corresponding normal experiment must be adjusted to reflect this effective TE. The image with the unshifted inversion pulse, i.e. the water and lipid vectors in phase, represents the sum of the water and fat magnetization. The image with the shifted inversion pulse, i.e. the water and lipid vectors 180.degree. out of phase, represents the difference between the water and fat magnetization. The sum and difference images each have medical diagnostic utility. Moreover, by summing or subtracting these two images, images of water only or fat only may be extracted.
The magnetic field of a magnetic resonance spectrometer is conventionally shimmed to improve the uniformity of the magnetic field. In one shimming technique, the magnetic field is manually or automatically adjusted until the distortion or the phase variation in an image of a phantom is minimized indicating an optimal field uniformity. In another technique, a magnetometer is positioned at a plurality of preselected locations in the image region and the strength of the maagnetic field measured. The field was selectively shimmed for greater uniformity.
The prior art techniques have various disadvantages such as ambiguities and inaccuracies in the determined magnetic field values. The shifted phase encoding techniques lack a sufficiently high signal-to-noise ratio for many purposes. They also do not account for patient related magnetic field inhomogeneities, particulalry the magnetic susceptibility of the patient. Further, the prior techniques cannot discriminate between field inhomogeneities and chemically shifted species.
The prior art techniques for magnetic field shimming suffer from various disadvantages. Direct magnetometer readings cannot account for patient magnetic susceptibility. They also offer only information about very gradual, i.e. low frequency component, changes in the field distribution. They are also time consuming and require special hardware for their implementation.
The prior techniques which encode magnetic field information in the magnetic resonance signal suffer from several disadvantages. First, they achieve only marginal signal-to-noise enhancement in the determinations. Second, they involve the utilization of phantoms which not only cannot reflect subject magnetic susceptibility but also contaminate the results with imformation about the phantom susceptibility. Third, the magnetic field representation so created is itself geometrically distorted and not an accurate field representation. Techniques of maximizing the delay time of the free induction delay (FID) performed on the subject or specimens are commonly utilized in high resolution spectroscopy. However, such techniques cannot discriminate chemical shift from magnetic field information. Accordingly, the intrinsic accurary is limited to the amount of chemical shift between water and lipid, approximately 3.5 ppm.
Prior art techniques for water/lipid separation which utilized phase encodement techniques require substantial increases in patient scanning time. Further, the phase encodement techniques are not corrected for magnetic field inhomogeneity problems. This limits their viability to special circumstances and prohibits the whole body utilization. These techniques also involve the utilization of a 180.degree. phase difference between water and lipid which causes ambiguities and cannot be used with inversion recovery scanning techniques.
The present invention contemplates a new and improved image encodement and reconstruction technique which overcomes the above reference disadvantages and others.